Caption: Gross specimens of nonsmoker's normal lung (left) and smoker's lung (right). The smoker's lung is darker, rougher and misshapen. Smoking damages the lungs in several ways. Constant irritation by tar in smoke causes chronic bronchitis, where the lung's airways are repeatedly inflamed. This thickens their walls and leads to a build up of fibrous scar tissue, reducing the lung's elasticity. Smoke also triggers the release of substances that make the lung's air sacs (alveoli) lose their walls and coalesce (emphysema). Lung cancer, which is caused by both nicotine and tar, leads to uncontrollable tumours that destroy lung tissue.